When objects move through a fluid media, drag is produced on all surfaces of the object. Common examples include aircraft moving through the air and ships moving through the sea. A similar mechanism occurs in pipelines where the fluid moves past the inner surface of the pipe. At least half of the drag for a commercial aircraft in the cruise condition is due to skin friction.
Skin friction is friction created when a fluid is in relative motion to a surface. Two general methods to reduce skin friction have been proposed. The first method is laminar flow control. This method takes advantage of the fact that friction is lower if the fluid is in a laminar flow condition than in a turbulent flow condition. The second method is turbulence control, where the turbulence is tamed or manipulated in such a way as to reduce the drag.
The most successful method for reducing turbulence's drag to date is the addition of tiny grooves parallel to the flow of fluid. This method confines incipent bursts of turbulence so that they cannot expand and disrupt the boundary surrounding an object in relative motion to a fluid. In order to understand the operation of the grooves, one must first understand the nature of the drag due to turbulence. To a large degree, drag is caused when an area of turbulent flow is found between the main flow of the fulid and the surface of the moving vehicle. Recent turbulent boundary layer research has clearly shown that the wall region is dominated by a sequence of eddy motions that are collectively called the bursting phenomenon. It begins with a pair of elongated, streamwise, counter-rotating vortices having diameters of approximately 40 .nu./u.sub..tau., where .nu./u.sub..tau. is the viscous length scale, .nu. is the kinematic viscosity and u is the friction velocity. These vortices exist in a strong shear and induce low- and high- speed regions between them as shown in Section 2--2 of FIG. 1 (prior art). The vortices and the accompanying eddy structures occur randomly in space and time. However, their appearance is regular enough that an average spanwise wavelength of approximately 80-100 .nu./u.sub..tau. has been identified. It is also observed that the low-speed regions grow downstream and develop inflectional U(y) profiles as sketched. At approximately the same time, the interface beteen the low-and high-speed fluid begins to oscillate. The low-speed region lifts up away from the wall as the oscillation amplitude increases and the flow rapidly breaks down into a completely random pattern. Since this latter process occurs on a very short time scale, it is referred to as a "burst". The low-speed regions are quite narrow, e.g., 20 .nu./u.sub..tau., and may also have significant shear in the spanwise direction.
The longitudinal grooves confine the bursts to a non-random pattern as well as reducing the rate at which they lift up from the wall. The result is a reduction in skin friction, or drag, of from 5-8%. This reduction has been insufficient to lead to wide-scale commercial application of the ridging method.
Another approach to reduction of turbulent skin friction drag is the provision of a porous surface of the object through which fluid is withdrawn to entirely remove the boundary layer. This method is fully described in U.S. Pat. No. 3,604,661 to Robert Alfred Mayer, Jr. While this method is effective to reduce skin friction it may not meet with widespread commercial application because the power required to produce a sufficient flow through the slots is in excess of the friction savings. Accordingly, there is a demand for an economical method of reducing skin friction.